The present invention relates generally to forming fiber reinforced plastic preforms and, more particularly, to a method and apparatus for controlling fiber deposition in a fiber reinforced preform.
Fiber reinforced plastic (FRP) parts or composite parts are well known and used in a wide variety of applications. An FRP part generally consists of a plastic shape in which carbon, fiberglass, or other reinforcing fibers are dispersed in order to provide strength to the component. One method of making an FRP part is known as resin transfer molding (RTM).
In RTM, fibrous material in a mold is injected with resin which cures to form the part. Examples of these techniques are disclosed in commonly assigned U.S. Pat. Nos. 4,740,346--Perimeter Resin Feeding of Composite Structures; 4,849,147--Method of Making a Molded Structure Having Integrally Formed Attachment Members; and 4,863,771--Hollow Fiber Reinforced Structure and Method of Making Same, each of which is hereby specifically incorporated by reference. In RTM, fibrous material is often formed into a preliminary shape before being placed into the mold. The shaped sections generally conform to the contour of adjacent mold die surfaces and are known as preforms. Preforms have been constructed using several different manufacturing approaches. One such approach is to direct chopped fibers by means of a flow of air onto a screen. One problem with this technique is that it is difficult to obtain desired fiber orientation. Another method utilizes mats of fibrous material to make the preforms. This method, however, produces undesirable amounts of scrap material and is labor intensive, thus resulting in production cost inefficiencies.
Still another technique, known as a wet slurry process, is disclosed, for example, in Keown et al. ("Wet Slurry Process Brings Precision To Reinforced Plastics"). Keown discloses a slurry containing chopped fibers drawn by vacuum into a chamber covered by a screen. As a result, the fibers are deposited on the screen. This approach, however, is associated with certain disadvantages. For example, it is difficult to consistently obtain the desired fiber orientation and compactness of the fibers using this equipment. In addition, the pumps and other equipment required to create the vacuum and draw the slurry through the screen may be unduly complex and difficult to maintain. Furthermore, the process is relatively slow.
An improved wet slurry process is disclosed in commonly assigned U.S. Pat. No. 5,039,465--Method and Apparatus For Forming Fiber Reinforced Plastic Preforms From a Wet Slurry, which is also hereby incorporated by reference. The process disclosed therein teaches creating a preform by raising a screen through a tank containing a slurry of fibers resulting in the fibers being deposited on the screen. While this approach is promising, it also has some drawbacks. For example, structural integrity of the preforms may be compromised as the screen is raised out of the liquid. As long as the screen is moving beneath the surface of the slurry, pressure from the slurry forces the fibers on the screen and holds them in position. However, as soon as the screen breaks the plane of the top of the slurry as the preform is being removed from the tank, the liquid and fiber mixture surrounding the screen tends to rush into the interior cavity of the preform. As the slurry rushes into the preform, the upright side walls of the preform may collapse thereby creating a need for costly repair work or discard of the entire preform. Another challenge in the construction of fiber reinforced preforms is that of maintaining uniform wall thickness throughout the preform. While drawing the screen through the slurry, more liquid is forced through the major surface of the screen that is perpendicular to the direction of draw than the upright side walls that are parallel to the direction of movement of the screen. Because the quantity of fiber deposited on the screen is proportional to the amount of liquid forced through the screen, preforms constructed in this manner may contain sections of non-uniform thickness.